The present invention relates to poly(alpha-olefin) plasticizers and plasticized polymers comprising such poly(alpha-olefin) plasticizers.
Polyolefins are used for making a wide variety of products. However, one drawback of some polyolefins is their relatively high glass transition temperature (Tg). Polypropylene homopolymers and copolymers are particularly problematic in this respect. This high Tg may result in materials that are difficult to process and which may be brittle, especially at low temperatures. Also, for many products, high molecular weight polyolefins are required to provide the desired polymer properties. High molecular weight polyolefins can be even more difficult to process, due to their high melt viscosities.
As a result, there is a need to provide polymers with good processability that are also able to maintain their advantageous properties over time when exposed to a wide temperature range.
One important property for such polymers is low-temperature toughness which should be improved while still providing a polymer that maintains its properties upon exposure to elevated temperatures over time.
Addition of a plasticizer to a polyolefin is known to improve properties such as impact strength and processability. Such plasticizers are often used to lower the Tg of the polymer. Lowering the Tg of the polymer can improve the processability and low temperature impact toughness of the polymer and reduce the tendency of the polymer to become brittle at low temperatures. In order to achieve this combination of advantages, the plasticizer should have the ability to maintain its viscosity at low temperature. Some plasticizers have a tendency to crystallize or form structure in the plasticizer at low temperatures leading to an undesirable increase in viscosity of the plasticizer. This may result in an adverse impact on the low temperature impact toughness of a polymer plasticized with this plasticizer by causing the polymer to become brittle.
Many plasticized polymers also suffer from problems such as blooming or diffusion of plasticizer to the surface of the polymer, or even volatilization of the plasticizer from the polymer, each resulting in long-term deterioration of polymer properties. As a result, plasticizers are sought which have a relatively low pour point and thus can impart the advantageous lowering of the Tg of the polymer discussed above while at the same time ensuring that the polymer retains its advantageous properties over time, particularly when exposed to high temperatures. In order to achieve this, the volatility or tendency of the plasticizer to diffuse to the surface of the polymer must be balanced with the lowering of the Tg of the polymer. One way to reduce the volatility of the plasticizer is to increase its number average molecular weight. However, increasing the number average molecular weight can have an adverse influence on the Tg of the plasticized polymer and the processability of the polymer.
Plasticizers having lower number-average molecular weights (Mn) tend to be better at lowering the Tg of the polymer since the Tg of the polymer tends to be inversely proportional to the Mn of the plasticizer. However, as the number-average molecular weight (Mn) of the plasticizer decreases, the plasticization durability of the properties imparted by the plasticizer to the polymer is adversely affected, especially when exposed to high temperatures. This is at least partially due to the increased tendency of the plasticizer to migrate to the surface of the polymer and/or volatilize. Thus, improved plasticizers are sought that have the ability to lower the Tg of a polymer without lowering the number average molecular weight of the plasticizer.
Another key aspect for selection of an appropriate plasticizer is the chemical compatibility of the plasticizer with the polymer. To achieve this goal, it is desirable to employ plasticizers that are chemically similar to the polymer that is being plasticized. This will enhance the chemical compatibility between the plasticizer and the polymer. For this purpose, polyalphaolefins (PAOs) have been proposed for use as polyolefin plasticizers.
Such PAOs are typically oligomers of olefins having five or more carbon atoms. In some cases, such oligomers with five or more carbon atoms may be copolymerized with ethylene to C4 olefins to reduce the pour point of the plasticizer. US 2004/054040 and WO 2004/014997 disclose PAO plasticizers having a weight average molecular weight (Mw) in the range of 100 to 20,000 g/mol. and a kinematic viscosity at 100° C. (KV100) of 0.1 to 3,000 cSt.
US 2004/106723 and WO 2004/014998 disclose plasticizers having a KV100 of 10 cSt or more and a viscosity index (VI) of 100 or more. These plasticizers include oligomers of C5 to C14 olefins. U.S. Pat. No. 4,536,537 discloses PAO plasticizers having a kinematic viscosity at 38° C. of about 2 to 6 cSt. WO 98/044041, WO 2002/018487 and WO 2003/048252 disclose PAO plasticizers having a KV100 of about 4 to 8 cSt.
WO 2009/020706 discloses plasticizers that are regularly branched PAO oligomers of one or more C2-C20 alpha-olefins having a KV100 of 3 to 3,000 cSt, a branching irregularity index (‘BII’) of 0.40 or less, and a molar-average carbon number (CLAO) of 6 to 14. Preferably, the regularly branched PAOs have a Mw/Mn of less than 2.3. Polyolefins plasticized with the regularly branched PAOs are said to have a reduced volatility as measured by thermogravimetric analysis, compared to comparable polyolefin compositions plasticized with conventional PAOs, or a reduced glass transition temperature (Tg) of the composition compared to comparable polyolefin compositions plasticized with conventional PAOs.
PAO plasticizers are typically prepared by olefin isomerization which results in oligomers having a variety of different structures caused by, for example, irregular branching. According to the present invention, it has now been found that PAOs having certain characteristics indicative of their structure can provide an improved combination of plasticization efficiency and plasticization durability of the properties imparted by the plasticizers when used in polyolefins.